A known hydrogen sensor is described in WO 2006/008253 A1 (Inficon). This hydrogen sensor has a membrane that is selectively permeable to hydrogen and closes off a pressure-tight housing. The housing contains a Penning pressure sensor with two parallel cathode plates between which an anode ring is arranged. A voltage source supplies a direct voltage that is applied between the cathode plates and the anode ring. The magnetic field required for a Penning-type discharge is generated by a permanent magnet provided outside the closed housing. A getter pump is connected to the housing, which draws hydrogen and reduces the hydrogen partial pressure. The detection chamber of the housing is connected to the getter pump through a throttling channel. If hydrogen from the atmosphere enters the detection chamber through the membrane, which is selectively permeable to hydrogen, the pressure in the detection chamber rises. Since the hydrogen reaches the getter pump only with a delay due to the throttling channel, the pressure in the detection chamber rises. This pressure increase is detected by the pressure sensor and is evaluated as an intrusion of hydrogen. Even under steady-state conditions, the partial pressure in the detection chamber is lower than the partial pressure of the surroundings.
In DE 100 31 882 A1 (Leybold) a sensor for helium or hydrogen is described which comprises a vacuum-tight housing with a selectively acting passage for the gas to be detected. The housing is made of glass and the selectively acting passage is a membrane of a silicon material on which a silicon disc with openings and a heating are arranged. The housing holds a Penning sensor. The housing is not connected to a vacuum pump. The Penning-type discharge of the Penning sensor additionally acts as a getter pump.
In J. Vac. Sci. Technol. A 26(3), May/June 2008 of the American Vacuum Society, pages 352-359, an article titled “On the gas species dependence of Pirani vacuum gauges” by Karl Jousten was published which describes the principle of the Pirani pressure sensor. This is an apparatus that measures the thermal conductivity of gases and thereby determines pressure. This benefits from the fact that thermal conductivity is proportional to pressure. The heat transmission from a heating element through surrounding gas to an enclosure of a more or less constant temperature is measured. The heating element is a part of a Wheatstone bridge operated such that the heating power required to maintain the temperature is measured. With other Pirani sensors, the heating power is maintained constant and the resistance or the temperature of the heating element is measured as the pressure indicator. Pirani pressure sensors have to be calibrated appropriately for the respective gas, since gas-dependent correction factors must be taken into account when evaluating the signals.